ASSESSMENT OF CURRENT RESULTS AND OUTLOOK ON FUTURE EFFORTS

QUANTUM COMMUNICATION

FIBER BASED SYSTEMS

Towards higher bit rates

1. Fast electronics, this includes fast sources and fast and low-loss phase modulators. This is mainly a (non-trivial) engineering problem.
2. Improved detectors: lower dark counts (<10-6 per ns), shorter dead times (<1 micros), less time-jitter (<100 ps) and higher detection efficiency (>15%). This is a non trivial solid state physics challenge.
3. Invent and investigate new protocols inspired by existing and reliable components, like "decoy states" [1] and the SARG protocol [2]. Also protocols based on fast homodyne detection methods can be thought of, such as the continuous variables protocols [3]. This is mainly a matter of the physicists' imagination!
4. It is known that existing classical communication procedures and security proofs do not make optimal use of the correlations that are generated in the physical set-up and can be improved. Further improvement in secure key rate can follow from a scenario of trusted sending and receiving devices which cannot be manipulated by an eavesdropper. It would also be valuable to have security proofs easier to understand for classical cryptographers.
5. Single-photon sources have made spectacular progress in the last years [4], but it is not clear yet whether they will be able to fulfill practical needs for high repetition rates, high coupling efficiency and electronic cooling (no liquid helium). It is not even necessary to use single photon sources since also QKD with weak laser pulses can be proven to be secure; see e.g. [5]. Moreover, the performance of ideal single photon sources can also be achieved using laser pulses with a phase reference, as has been proven by a recent analysis by Koashi [6]. Fourier-transform limited single-photon sources with negligible time-jitter could also be used as building blocks for linear optics quantum computing.
6. Quantum communication with entangled states will be important to further develop quantum teleportation and entanglement swapping in view of their possible use in connection future quantum computers.

Europe has a leading role for point 6, while it is competing with the US for 3 and 4 (and also with Japan for 5). US and Japan are ahead concerning 1 and 2.

Towards longer distances

In today's system the distance is limited by the fiber loss and the detector dark-counts: at large distances the dark-counts dominate the signal. To improve the distance one can, from the simplest and less effective to the most challenging and most effective:

1. Improve the detectors: lower dark counts automatically increase the distance. However, the bit rate decreases exponentially with distance.
2. Improve the fibers: air-core photonic band-gap fibers have the potential to surpass silica fibers. (Even pure silica core photonic bandgap fibers could improve on today's telecom fibers, but only by at most 0.05 dB/km). This is a tremendous engineering challenge, with applications which would impact the whole field of optical telecommunications=
3. Use quantum relays exploiting quantum teleportation and entanglement swapping [7]. Dividing the connection in sections allows one to open the receiving detector less frequently, lowering thus the dark-count rate. For any given detector efficiency, this allows one to gain a factor of about 5 in distance. But the maximal distance is still limited and the bit rate still decreases exponentially with distance. Quantum relays require entangled photon sources. It should be stressed that quantum relays are anyway necessary for quantum repeaters. Today's longest distance demonstration is a quantum teleportation lab experiment connecting three 2 km long sections. The next crucial milestone in this direction will be a field demonstration over tens of km of entanglement swapping.
4. Use quantum repeaters: fully developed quantum repeaters have the potential of extending quantum communication to arbitrary long distances with a constant bit rate [8]. It is extremely challenging physics and still basic research. A quantum repeater requires a quantum memory. The latter has to outperform an optical fiber delay loop. This important milestone is described in section 4.1.3.

In this subfield, Europe is presently the leader; except for the first one, where the US and Japan are ahead.

Quantum continuous variables

Besides qubits, quantum continuous variables (QCV) have emerged as a new tool for developing novel quantum communication and information processing protocols. Encoding quantum continuous information into the quadrature of a light mode, or into the collective spin variable of a mesoscopic atomic ensemble, has proven to be a very interesting alternative to the standard concept of quantum bits. Several experimental breakthroughs have been achieved recently demonstrating this concept, namely the quantum teleportation of a coherent state, the preparation of distant entangled atomic ensembles, or the implementation of a quantum key distribution scheme relying on coherent states Beyond these major experimental results, a large number of theoretical ideas have appeared in the literature, proposing to use QCV for achieving dense coding, entanglement purification or distillation, error correcting codes, cloning or telecloning, memories based on light-atoms interfaces, etc. In addition, some fundamental studies have been carried out on the entanglement of multimode Gaussian states, or on the capacity of Gaussian quantum channels.

These results are stimulating more research work, with many theoretical and experimental developments, especially in the directions of improved and/or novel quantum communication and secret sharing protocols, quantum memories and quantum repeaters using the light-atoms quantum interface, and the use of squeezed, or entangled, or even non-Gaussian states of light in order to make some new information processing with continuous variables possible.

New applications and protocols

The field of quantum communication is still very young, having been essentially unknown until 10 years ago. One should expect new ideas and leave open space for basic research. From the theoretical point of view, there are several problems that have to be considered in the context of quantum communication. First of all, since the field is still very young, one should expect new applications related to both the efficiency as well as the secrecy in communications. Examples of the first can be connected to secret voting protocols, digital signatures, or fingerprinting. Examples of the second field could be, for example, connected to dense coding, or agenda protocols. Apart from that, there are still several theoretical open questions of crucial importance for quantum cryptography. They are related to the tolerance to noise of current protocols (both with one and two way communication), the connection between single photon and continuous variable protocols, and the search for more efficient and fast ways of distributing keys.

Quantum communication protocols can be often understood as entanglement manipulation protocols. An important class of these protocols delivers classical data with properties derived from the underlying quantum state. For this class the question arises whether one can replace the quantum manipulation and subsequent measurement by another two-step procedure that first measures the quantum states and then performs classical communication protocols on the resulting data to complete the task. Such an implementation would be preferential in real implementation, as is illustrated in the case of quantum key distribution. It is important to study under which circumstances such a replacement can be done.

Key references

[1] W.-Y. Hwang, Quantum Key Distribution With High Loss: Towards Global Secure Communications, Phys. Rev. Lett. 91, 057901 (2003).

[2] V. Scarani, A. Acín, G. Ribordy, and N.s Gisin, Quantum Cryptography Protocols Robust against Photon Number Splitting Attacks for Weak Laser Pulse Implementations, Phys. Rev. Lett. 92, 057901 (2004).

[3] F. Grosshans, G. Van Assche, J. Wenger, R. Brouri, N. J. Cerf and Philippe Grangier, Quantum key distribution using gaussian-modulated coherent states, Nature 421, 238 (2003).

[4] A. Beveratos, R. Brouri, T. Gacoin, A. Villing, J.-P. Poizat, and P. Grangier, Single Photon Quantum Cryptography, Phys. Rev. Lett. 89, 187901 (2002); E. Waks, K Inoue, C. Santori, D. Fattal, J. Vuckovic, G. S. Solomon, and Y. Yamamoto, Quantum cryptography with a photon turnstile device, Nature 420, 762 (2002).

[5] H. Inamori, N. Lütkenhaus, D. Mayers, Unconditional Security of Practical Quantum Key Distribution, quant-ph/0107017, http://xxx.arxiv.org.

[6] M. Koashi, Unconditional Security of Coherent-State Quantum Key Distribution with a Strong Phase-Reference Pulse, Phys. Rev. Lett. 93, 120501 (2004).

[7] B. C. Jacobs, T. B. Pittman, and J. D. Franson, Quantum relays and noise suppression using linear optics, Phys. Rev. A 66, 052307 (2002); D. Collins, N. Gisin, H. D. Riedmatten, Quantum relays for long distance quantum cryptography, Journal of Modern Optics 52, 735-753 (2005).

[8] H.-J. Briegel, W. Dür, J. I. Cirac, and P. Zoller, Quantum Repeaters: The Role of Imperfect Local Operations in Quantum Communication, Phys. Rev. Lett. 81, 5932 (1998).